1. Field of the Invention
This invention relates to a metallized high specular gloss polyethylene plexifilamentary film-fibril sheet with low emissivity, high moisture vapor permeability and good resistance to air and water penetration, and a process to prepare the high specular gloss and metallized sheets.
2. Description of the Prior Art
Materials with low emissivities have been used as thermal barriers to reduce the amount of heat transfer by radiation in many areas of construction. The lower the emissivity value, the better the barrier by virtue of reflecting infrared radiation. Specifically, a blackbody reference, of emissivity equal to 1, is capable of absorbing all radiant energy. A non-transmitting barrier with an emissivity of 0.1, for example, means that 10 percent of the radiant energy directed at it is absorbed and only that amount can be emitted to its surroundings. That is because, by definition, 90 percent of the radiant energy is reflected away.
In the past, bright aluminum foils or metallized films have been the most effective products due to their low emissivities and ease of installation. Low emissivity materials are typically referred to as "Radiant Barriers". Installation of radiant barriers in residential dwellings is usually in one of three locations in the attic space; (1) on the attic floor above the insulation installed over the floor joists, (2) under the roof rafters, or (3) draped under the roof deck. Other installation locations exist in vaulted ceiling arrangements and behind vertical knee walls.
In some installation locations, the radiant barrier may serve a dual energy saving function. That is, reducing the radiation heat transfer from the roof in the summer months and reducing convective heat losses in the winter months by restricting air exchange in and around installed insulation. However, in dwellings and other buildings, moisture migrates from the living space, creating a requirement for the radiant barrier to be able to transmit moisture. Aluminum foils and metallized films inherently are vapor barriers and are unsuitable for these locations. To overcome this moisture barrier problem in the radiant barriers of the prior art, perforations have been made in these products to increase their ability to transmit moisture. This is done at the expense of increasing the emissivity and reducing effectiveness as an air barrier.
Nonwoven sheets made from polyethylene film-fibrils have been known to provide a combination of high moisture vapor permeability and good resistance to air penetration. They have been used in this capacity as an air infiltration barrier housewrap product. Product Licensing Index, Research Disclosure, "Tyvek Air Infiltration Barrier In Housing Construction," p. 556 (Oct., 1979). This type of nonwoven sheet has a high emissivity, generally about 0.55, and is not suitable as a radiant barrier. Polyethylene plexifilamentary film-fibril sheets such as "Tyvek" (Registered Trademark of E.I. du Pont de Nemours & Co. Inc., Wilmington, Del.) Types 10, 1621C and 1622 which have been metallized are commercially available. Such metallized sheets have, for example, been used for screening commercial glass-houses as disclosed in U.S. Pat. No. 4,508,776. These metallized polyethylene sheets of the prior art have high emissivities, on the order of 0.2 to 0.32, and are generally unsuitable for use as radiant barriers.